For an increasing number of radio frequency applications, particularly in the microwave region, fabricating circuit devices using MMIC (monolithic microwave integrated circuit) techniques presents significant advantages in terms of cost and reliability.
Some microwave devices are difficult to implement monolithically, typically because of the size constraints inherent in integrated circuit fabrication. In particular, microwave devices requiring transmission line components have proved difficult to fabricate because the transmission line components often require series and/or shunt connections that cannot be achieved using conventional monolithic planar fabrication techniques. In particular, the conventional microstrip technique for monolithically fabricating planar transmission lines cannot be used to fabricate transmission line components with series/shunt connections.
An alternative design approach is to use non-planar coaxial transmission lines for the transmission line components. However, coaxial structures are cumbersome to integrate with MMIC components. Another alternative design uses suspended microstrip techniques in which a conductor is suspended over a surface separated by an air dielectric gap. However, suspended substrate techniques are impractical to implement monolithically due to the circuit area required and the fragility of the typical GaAs substrate material.
An example of a microwave device that is difficult to synthesize in MMIC is a passive balun. While transformer hybrids are common at lower frequencies, as the frequency of operation extends into the microwave region (above several GHz), transformer hybrids can no longer be economically fabricated. At these frequencies, transmission line passive baluns are the only practical solution. However, transmission line baluns typically involve series and shunt connected transmission line components with different lengths and impedances to achieve flexibility in matching. As a result, transmission line baluns have heretofore not been integrated into monolithic MMIC designs.
Accordingly, a need exists for a planar transmission line that can be fabricated monolithically with series/shunt connected components, allowing integration into an MMIC device.